A substance that exhibits photochromism is expected to be applied to various industrial products such as an optical disc (ultrahigh-density memory), an optical switch, optical printing ink, a display, sunglasses, and light control glass. As used herein, the photochromism refers to a property of exhibiting a kind of a reversible phenomenon, i.e., a property of undergoing a color change from an original color upon application of light but undergoing color fading to return to the original color upon application of another type of light or application of heat. A substance that exhibits such a property is referred to as a photochromic substance.
As the photochromic substance, both a photochromic substance made of an organic substance and a photochromic substance made of an inorganic substance are known. Known examples of the photochromic substance made of an organic substance encompass azobenzene, spiropyran, and diarylethene. However, the photochromic substance made of an organic substance has problems such as a low stability against heat and liability to undergo a chemical degradation due to repeated isomerization.
Therefore, a photochromic substance made of an inorganic substance, which has chemical and thermal stability, is attracting attention. Known examples of the photochromic substance made of an inorganic substance encompass molybdenum oxide, tungsten oxide, niobium oxide, titanium oxide, a mixture of these oxides, and glass in which a silver halide is included. However, these substances essentially exhibit sensitivity only in an ultraviolet light region which is harmful to human bodies. Moreover, there are only a small variety of commercially available small light source emitting ultraviolet light. Use of these substances therefore undesirably limits the field of application.
In view of the circumstances, recent years have witnessed an increase of research on photochromic substances which allow use of a small semiconductor laser, LED, or the like as a light source, especially photochromic substances each of which is made of an inorganic substance and which exhibits sensitivity in a visible light region so as to enable development of, for example, a small optical disc (ultrahigh-density memory).
For example, Patent Literature 1 describes a photochromic substance, made of divanadium pentoxide, which exhibits photochromism upon application of visible light. However, the photochromic substance described in Patent Literature 1, i.e., a conventional photochromic substance made of an inorganic substance has problems of insufficient durability and slow response speed. Moreover, this photochromic substance has a problem of poor reversibility because it can no longer return to its original color after several irradiations of light. Furthermore, there is another problem of strong toxicity of divanadium pentoxide.
In view of this, Patent Literature 2 proposes a photochromic substance that can be produced by preparing barium magnesium silicate in a reductive atmosphere and that exhibits good sensitivity in a visible light region. Patent Literature 2 also describes that photochromism can be improved by adding a specific metal element to barium magnesium silicate.
FIG. 3 is a graph showing, for each metal element added to barium magnesium silicate, reflectance of light having a wavelength of 523 nm from the metal element-added barium magnesium silicate after irradiation of light having a wavelength of 365 nm (reflectance obtained by changing M″ in Ba2Mg2.5Si2O8.25: M″0.001). As is clear from FIG. 3, barium magnesium silicate is lowered in reflectance and exhibits a high photochromic property as a result of addition of especially iron (Fe) or europium (Eu). Since iron (Fe) is easily available unlike rare earthes such as europium (Eu), use of iron (Fe) makes it possible to cut down a production cost of a photochromic substance.